When images are formed using electrophotographic image forming apparatus (such as optical printers, digital copiers and optical plotters), a light source using laser is typically used for the image forming apparatus. Such image forming apparatus include a photoreceptor drum serving as an image bearing member, and an optical scanning device having a light source to emit light flux (scanning light flux), a light deflector to deflect the emitted light flux to scan a surface (scanning surface) of the photoreceptor drum, thereby forming an electrostatic latent image on the surface of the photoreceptor drum. The thus formed electrostatic latent image is typically developed with a developer including a toner to form a toner image on the surface of the photoreceptor drum.
In the optical scanning device, the light flux emitted by the light source to scan the surface of the photoreceptor drum is modulated by image information of an original image, and therefore an electrostatic latent image corresponding to the original image is formed on the surface of the photoreceptor drum.
In such image forming apparatus, the light quantity of the light flux emitted by the light source tends to vary when the environmental temperature changes and/or after the image forming apparatus are repeatedly used for a long period of time. In this case, the potential of the electrostatic latent image also varies, thereby causing an uneven image density problem in that the image density of the output image (toner image) has uneven image density.
In attempting to prevent occurrence of the uneven image density problem, published unexamined Japanese patent applications Nos. 2006-332142, 2010-217353 and 2008-268683 have disclosed optical scanning devices performing an APC (Auto Power Control) operation such that part of light emitted by a light source is received as monitoring light by a photodetector such as a photodiode, and the output level of the light source is controlled based on the detection result.
However, it is difficult for the optical scanning devices to stably perform a scanning operation on a scanning surface without increasing the costs of the optical scanning devices.
In an optical scanning device using an edge emitting laser for the light source, the APC operation can be performed using light emitted backward by the edge emitting laser as monitoring light. However, a surface emitting laser cannot emit light backward unlike an edge emitting laser, and therefore it is difficult to provide an optical scanning device having a light source equipped with a monitoring photodetector. When a surface emitting laser is used for a light source, a light quantity controlling method in which part of light flux emitted by the surface emitting laser is branched using an optical element such as a beam splitter and a half mirror so as to be guided to a monitoring photodetector, and the APC operation is performed using the output from the photodetector is typically used.
Specifically, in attempting to perform light quantity controlling, a monitoring photodetector in which an aperture is arranged between a half mirror serving as an optical branching element and a collimator lens is proposed. It is described therein that by providing such an aperture, the ratio of the light quantity of a laser beam proceeding toward a photoreceptor drum after passing the half mirror to the light quantity of a laser beam proceeding toward the photodetector after reflected from the half mirror does not change, and therefore a stable APC operation can be performed.
In addition, a multi-beam light source device is proposed which includes a light source in which multiple light emitters are monolithically arranged in a main scanning direction; a coupling lens to convert the multiple light beams emitted by the multiple light emitters so as to have a predetermined focused state; and a supporter to integrally support the light source and the coupling lens. In this multi-beam light source device, the supporter includes a first supporting member which supports the coupling lens and which is rotatable on the optical axis of light beams emitted by the light source, and a second supporting member which supports the light source and which is set such that inclination of the first supporting member at the main scanning cross-section is adjustable.
Further, a monitoring device for detecting the light quantity of light flux emitted by a vertical cavity surface emitting laser (VCSEL) light source is proposed. Specifically, the monitoring device includes an aperture in which a main portion of light flux emitted by the VCSEL light source and having the largest light intensity passes through the center of the aperture; an optical separation element which reflects light incident to the vicinity of the aperture as a monitoring light reflux; a second aperture which restricts the diameter of the monitoring light reflux; and a photodetector to detect the monitoring light reflux passing through the second aperture.
However, the monitoring photodetector mentioned above has a drawback in that the length of the light path between the light source and the photodetector is long, and therefore the device has a large size.
In addition, the multi-beam light source device mentioned above has a drawback in that the device is seriously affected by variation of the divergence angle of light.
The monitoring device mentioned above has good light utilization efficiency because of performing monitoring using light, which is not used for image writing (scanning). In addition, since the second aperture is provided, deterioration of the light quantity detection precision caused by variation of the divergence angle of light can be prevented. However, the device is constituted of many parts such as the light source, the first aperture (light path branching member), the second aperture, a light receiving lens, and the photodetector, and therefore the device has high manufacturing costs while having a large size because the length of the light path between the light source and the photodetector is long. In addition, it is necessary to adjust the positions of the light receiving lens and the photodetector with high precision such that the monitoring light flux is satisfactorily focused on a proper position of the photodetector.
For these reasons, the inventors recognized that there is a need for an optical scanning device which can stably scan a surface of an object without increasing the costs thereof. In addition, there is a need for a light source which can detect light quantity with high precision without increasing the size thereof.